you cast detect magic on a spellcaster, doesn't matter what kind, druid, bard, sorcerer, cleric, wizard, even a paladin or ranger high enough to have spells. the person you cast on isn't carrying anything magicl, isn't casting a spell, isn't currently affected by a spell. does detect magic tell you anything? what if you cast it on a race with innate magical ability like a gnome?

3E takes a very mechanistic view of magic, since anyone can be a caster, etc. I think that spell casting ability in the implied setting wouldn't be detectable as any sort of aura. It'd probably be a mind-affecting spell that detects memories of casting or something.

However, I agree that it'd be a neat feature of a game world - just be sure you warn your players before they make their wizard characters. :)

The way I understand it, magic and magic ability emanates from a being's lifeforce. Some people have more magic ability than others. Imagine seeing infared vision. Those with magical ability would light up more so than Joe Schmoe with little to none.

I don't know how you want to define it though. That's the way it's always been done in my campaigns. You're the DM. You can make the spell work how you want it. The story is what's important.

One way I've done it in my game is that any spellcaster can "cast" detect magic at any time by "opening their third eye." And when they do so, the third eye of others shows up as a radiant glimmering on the spellcaster's forehead. But strictly, by-the-rules, it doesn't show anything.